Method for the secondary cross-linking of hydrogels with N-acyl-2-oxazolidinones

ABSTRACT

Liquid-absorbent polymers are prepared by a process for the gel and/or surface postcrosslinking of water-absorbent polymers by the polymer being treated with a surface postcrosslinking solution and being postcrosslinked and dried during and after the treatment by raising the temperature, wherein the surface postcrosslinking solution comprises as crosslinker a compound of the formula I                    
     where 
     R 1 , R 2 , R 3  and R 4  are independently hydrogen, C 1 -C 12 -alkyl, C 1 -C 12 -alkenyl, C 6 -C 12 -aryl or C 1 -C 12 -alkoxy, 
     R 5  is C 1 -C 16 -alkyl, C 1 -C 4 -hydroxyalkyl, C 6 -C 12 -aryl, C 1 -C 16 -alkenyl or a group of the formula —(CH 2 ) n —COOH or —(CH 2 ) n —SO 3 H in the form of the acid or a respective metal salt, ammonium salt or triethanolammonium salt thereof or —(CH 2 —CH 2 —O—) m —R 6 , 
     R 6  is hydrogen or C 1 -C 16 -alkyl, 
     n is from 1 to 10, 
     m is from 1 to 20 
     dissolved in an inert solvent, 
     and are used in hygiene articles, packaging materials and nonwovens.

The present invention relates to a process for the gel or surfacepostcrosslinking of water-absorbent hydrogels withN-acyl-2-oxazolidinones, the polymers thus obtainable and their use inhygiene articles, packaging materials and nonwovens.

Hydrophilic, highly swellable hydrogels are in particular polymers of(co)polymerized hydrophilic monomers, graft (co)polymers of one or morehydrophilic monomers on a suitable grafting base, crosslinked celluloseor starch ethers, crosslinked carboxymethylcellulose, partly crosslinkedpolyalkylene oxide or natural products that are swellable in aqueousfluids, for example guar derivatives. Such hydrogels are used asproducts for absorbing aqueous solutions in the manufacture of diapers,tampons, sanitary napkins and other hygiene articles, and as waterretainers in market gardening.

To improve application properties, for example diaper rewet andabsorbency under load (AUL), hydrophilic, highly swellable hydrogels aregenerally surface or gel postcrosslinked. This postcrosslinking ispreferably carried out in the aqueous gel phase or as surfacepostcrosslinking of the ground and classified polymer particles.

Useful crosslinkers for this purpose include compounds containing atleast two groups capable of entering covalent bonds with the carboxylgroups of the hydrophilic polymer. Useful compounds include for exampledi- or polyglycidyl compounds, such as diglycidyl phosphonate,alkoxysilyl compounds, polyaziridines, polyamines or polyamidoamines,and these compounds can also be used in mixtures with each other (seefor example EP-A-0 083 022, EP-A-0 543 303 and EP-A-0 530 438).Polyamidoamines useful as crosslinkers are described in EP-A-0 349 935in particular.

A major disadvantage of these crosslinkers is their high reactivity,since it necessitates particular precautions in production to avoidundesirable side effects. Moreover, the aforementioned crosslinkers haveskin-irritating properties, which makes their use in hygiene articlesproblematical.

Known crosslinkers also include polyfunctional alcohols. For instance,EP-A-0 372 981, U.S. Pat. No. 4,666,983 and U.S. Pat. No. 5,385,983teach the use of hydrophilic polyalcohols and the use of polyhydroxysurfactants. The reaction is carried out at 120-250° C. The process hasthe disadvantage that the esterification which leads to crosslinking isvery slow even at such temperatures.

Prior German Patent Application DE-A-19 807 502 describes a process forpostcrosslinking with 2-oxazolidinones.

It is an object of the present invention to provide gel or surfacepostcrosslinking equivalent to or superior to the prior art by usingrelatively inert compounds capable of reacting with carboxyl groups.This object is to be achieved with a very short reaction time and a verylow reaction temperature.

We have found that this object is achieved, surprisingly, whenN-acyl-2-oxazolidinones are used as crosslinkers. More particularly, themoderate reactivity of the crosslinkers can be boosted with inorganic ororganic acidic catalysts. Useful catalysts include known inorganicmineral acids, their acidic salts with alkali metals or ammonium andalso their corresponding anhydrides. Useful organic catalysts includeknown carboxylic acids, sulfonic acids and amino acids.

The invention accordingly provides a process for the gel and/or surfacepostcrosslinking of water-absorbent polymers by the polymer beingtreated with a surface postcrosslinking solution and beingpostcrosslinked and dried during and after the treatment by raising thetemperature, wherein the surface postcrosslinking solution comprises ascrosslinker a compound of the formula I

where

R¹, R², R³ and R⁴ are independently hydrogen, C₁-C₁₂-alkyl,C₁-C₁₂-alkenyl, C₆-C₁₂-aryl or C₁-C₁₂-alkoxy,

R⁵ is C₁-C₁₆-alkyl, C₁-C₄-hydroxyalkyl, C₆-C₁₂-aryl, C₁-C₁₆-alkenyl or agroup of the formula —(CH₂)_(n)—COOH, —(CH₂)_(n)—SO₃H or—(CH₂—CH₂—O—)_(m)—R⁶, and

R⁶ is hydrogen or C₁-C₁₆-alkyl,

n is from 1 to 10,

m is from 1 to 20,

dissolved in an inert solvent.

When R⁵ is —(CH₂)_(n)—COOH or —(CH₂)_(n)—SO₃H, the formula I crosslinkerdescribed can also be present in ionic form, preferably as sodium orpotassium salt. The crosslinker may also be present in the form offurther metal salts, for example with Al³⁺, Mg²⁺, Li⁺, Ca²⁺, La³⁺,Ti^(2+/4+), Zn²⁺ or Fe^(2+/3+). It can also be present in the form ofammonium salts or triethanolammonium salts. A particularly preferredcrosslinker of the formula I is N-acetyl-2-oxazolidinone.

The postcrosslinking and drying temperature is preferably 50-250° C.,especially 50-200° C., most preferably 100-180° C. The surfacepostcrosslinking solution is preferably sprayed onto the polymer insuitable spray mixers. Following spray application, the polymer powderis dried thermally, and the crosslinking reaction can take place notonly before but also during the drying. Preference is given to sprayapplication of a solution of the crosslinker in reaction mixers ormixing and drying systems such as, for example, Lödige mixers, BEPEX®mixers, NAUTA® mixers, SHUGGI® mixers or PROCESSALL®. Moreover,fluidized-bed dryers may also be used.

Drying may take place in the mixer itself, by heating the outer casingor by blowing hot air in. It is similarly possible to use a downstreamdryer such as a tray dryer, a rotary tube dryer or a heatable screw. Butit is also possible, for example, to use an azeotropic distillation as adrying technique. The preferred residence time at this temperature inthe reaction mixer or dryer is less than 60 minutes, particularlypreferably less than 30 minutes.

In a preferred embodiment of the invention, the reaction is speeded byadding an acidic catalyst to the surface postcrosslinking solution.Useful catalysts for the process of the invention include all inorganicacids, their corresponding anhydrides, and organic acids. Examples areboric acid, sulfuric acid, hydroiodic acid, phosphoric acid, tartaricacid, acetic acid and toluenesulfonic acid. More particularly theirpolymeric forms, anhydrides and also the acidic salts of the polybasicacids are also suitable. Examples of these are boron oxide, sulfurtrioxide, diphosphorus pentoxide and ammonium dihydrogenphosphate.

The crosslinker is dissolved in inert solvents. The crosslinker is usedin an amount of from 0.01 to 5.0%, preferably from 0.05 to 0.5%, byweight, based on the polymer used. The preferred inert solvent is wateror a mixture of water with mono- or polyhydric alcohols. However, it isalso possible to use any unlimitedly water-miscible organic solventwhich is not itself reactive under the process conditions. When analcohol-water mixture is used, the alcohol content of this solution isfor example 10-90% by weight, preferably 30-70% by weight, especially40-60% by weight. Any alcohol of unlimited miscibility with water can beused, as can mixtures of two or more alcohols (eg.methanol+glycerol+water). The alcohol mixtures may contain the alcoholsin any desired mixing ratio. However, it is particularly preferable touse the following alcohols in aqueous solution: methanol, ethanol,isopropanol, ethylene glycol and particularly preferably 1,2-propanedioland 1,3-propanediol.

In a further preferred embodiment of the invention, the surfacepostcrosslinking solution is used in a ratio of 1-20% by weight, basedon the mass of the polymer. Particular preference is given to a solutionquantity of 0.5-10% by weight, based on the polymer.

The invention further provides crosslinked water-absorbent polymers thatare obtainable by the process according to the invention.

The hydrophilic, highly swellable hydrogels to be used in the process ofthe invention are in particular polymers of (co)polymerized hydrophilicmonomers, graft (co)polymers of one or more hydrophilic monomers on asuitable grafting base, crosslinked cellulose or starch ethers ornatural products swellable in aqueous fluids, for example guarderivatives. These hydrogels are known to one skilled in the art and aredescribed for example in U.S. Pat. No. 4,286.082, DE-C-27 06 135, U.S.Pat. No. 4,340,706, DE-C-37 13 601, DE-C-28 40 010, DE-A-43 44 548,DE-A-40 20 780, DE-A-40 15 085, DE-A-39 17 846, DE-A-38 07 289, DE-A-3533 337, DE-A-35 03 458, DE-A-42 44 548, DE-A-42 19 607, DE-A-40 21 847,DE-A-38 31 261, DE-A-35 11 086, DE-A-31 18 172, DE-A-30 28 043, DE-A-4418 881, EP-A-0 801 483, EP-A-0 455 985, EP-A-0 467 073, EP-A-0 312 952,EP-A-0 205 874, EP-A-0 499 774, DE-A 26 12 846, DE-A-40 20 780, EP-A-0205 674, U.S. Pat. No. 5,145,906, EP-A-0 530 438, EP-A-0 670 073, U.S.Pat. No. 4,057,521, U.S. Pat. No. 4,062,817, U.S. Pat. No. 4,525,527,U.S. Pat. No. 4,295,987, U.S. Pat. No. 5,011,892, U.S. Pat. No.4,076,663 or U.S. Pat. No. 4,931,497. The content of the aforementionedpatent documents is expressly incorporated herein by reference. Examplesof hydrophilic monomers useful for preparing these hydrophilic, highlyswellable hydrogels are polymerizable acids, such as acrylic acid,methacrylic acid, vinylsulfonic acid, vinylphosphonic acid, maleic acidincluding its anhydride, fumaric acid, itaconic acid,2-acrylamido-2-methylpropanesulfonic acid,2-acrylamido-2-methyl-propanephosphonic acid and its amides,hydroxyalkyl esters and amino- or ammonium-containing esters and amidesand also the alkali metal and/or ammonium salts of monomers containingacid groups. Also water-soluble N-vinylamides such as N-vinylformamideor else diallyldimethylammonium chloride. Preferred hydrophilic monomersare compounds of the general formula II

where

R⁷ is hydrogen, methyl or ethyl,

R⁸ is —COOR¹⁰, hydroxysulfonyl or phosphonyl, a(C₁-C₄)-alkanol-esterified phosphonyl group or a group of the formulaIII

R9 is hydrogen, methyl, ethyl or carboxyl,

R¹⁰ is hydrogen, amino-(C₁-C₄)-alkyl or hydroxy-(C₁-C₄)-alkyl, alkalimetal or ammonium ion and

R¹¹ is hydroxysulfonyl, phosphonyl or carboxyl

or an alkali metal or ammonium salt of each of these.

Examples of C₁-C₄-alkanols are methanol, ethanol, n-propanol,isopropanol or n-butanol.

Particularly preferred hydrophilic monomers are acrylic acid andmethacrylic acid and also their alkali metal and ammonium salts, forexample sodium acrylate, potassium acrylate or ammonium acrylate.

Useful grafting bases for hydrophilic hydrogels obtainable by, graftcopolymerization of olefinically unsaturated acids or their alkali metalor ammonium salts may be of natural or synthetic origin. Examples arestarch, cellulose or cellulose derivatives and also otherpolysaccharides and oligosaccharides, polyalkylene oxides, in particularpolyethylene oxides and polypropylene oxides, and also hydrophilicpolyesters.

Useful polyalkylene oxides have for example the formula IV

where

R¹² and R¹³ are independently hydrogen, alkyl, alkenyl or aryl,

x is hydrogen or methyl, and

n is an integer from 1 to 10,000.

R¹² and R¹³ are each preferably hydrogen, (C₁-C₄)alkyl, (C₂-C₆)alkenylor phenyl.

Preferred hydrogels are in particular polyacrylates, polymethacrylatesand also the graft polymers described in U.S. Pat. No. 4,931,497, U.S.Pat. No. 5,011,892 and U.S. Pat. No. 5,041,496.

The hydrophilic, highly swellable hydrogels are preferably incrosslinked form; that is, they include compounds having at least twodouble bonds which have been copolymerized into the polymer network.Suitable crosslinkers are in particular N,N′-methylene-bisacrylamide,N,N′-methylenebismethacrylamide, esters of unsaturated mono- orpolycarboxylic acids of polyols, such as diacrylate or triacrylate,examples being the diacrylates and dimethacrylates of butanediol and ofethylene glycol, and trimethylolpropanetriacrylate, and also allylcompounds such as allyl (meth)acrylate, triallyl cyanurate, diallylmaleate, polyallyl esters, tetraallyloxyethane, triallylamine,tetra-allylethylenediamine, allyl esters of phosphoric acid and alsovinylphosphonic acid derivatives as described for example in EP-A-0 343427. In the process of the invention, however, particular preference isgiven to hydrogels prepared using polyallyl ethers as crosslinkers andby acidic homopolymerization of acrylic acid. Suitable crosslinkers arepentaerythritol tri-and tetraallyl ether, polyethylene glycol diallylether, monoethylene glycol diallyl ether, glycerol di- and triallylether, polyallyl ethers based on sorbitol and also ethoxylated variantsthereof.

The water-absorbent polymer is preferably a polymeric acrylic acid or apolyacrylate. This water-absorbent polymer may be prepared by a processknown from the literature. Preference is given to polymers containingcrosslinking comonomers in amounts of 0.001-10 mol %, preferably 0.01-1mol %, but very particular preference is given to polymers obtained byfree-radical polymerization using a polyfunctional ethylenicallyunsaturated free-radical crosslinker which additionally bears at leastone free hydroxyl group (eg. pentaerythritol triallyl ether ortrimethylolpropane diallyl ether).

The hydrophilic, highly swellable hydrogels are preparable byconventional polymerization processes. Preference is given to additionpolymerization in aqueous solution by the process known as gelpolymerization. In this process from 15 to 50% strength by weightaqueous solutions of one or more hydrophilic monomers and optionally ofa suitable grafting base are polymerized in the presence of afree-radical initiator, preferably without mechanical mixing, utilizingthe Trommsdorff-Norrish effect (Makromol. Chem. 1, 169 (1947)). Thepolymerization reaction may be carried out in the temperature range from0° C. to 150° C., preferably from 10° C. to 100° C., not only atatmospheric pressure but also at elevated or reduced pressure. Ascustomary, the polymerization may also be carried out in a protectivegas atmosphere, preferably under nitrogen. The polymerization may beinitiated using high-energy electromagnetic radiation or the customarychemical polymerization initiators, for example organic peroxides, suchas benzoyl peroxide, tert-butyl hydroperoxide, methyl ethyl ketoneperoxide, cumene hydroperoxide, azo compounds such asazodiisobutyronitrile and also inorganic peroxy compounds such as(NH₄)₂S₂O₈, K₂S₂O₈ or H₂O₂. They may if desired be used in combinationwith reducing agents such as sodium hydrogensulfite and iron(II) sulfateor redox systems where the reducing component is an aliphatic oraromatic sulfinic acid, such as benzenesulfinic acid or toluenesulfinicacid or derivatives thereof, such as Mannich adducts of sulfinic acids,aldehydes and amino compounds as described in DE-A-1 301 566. Thequalities of the polymers may be further improved by postheating thepolymer gels for a number of hours within the temperature range from 50to 130° C., preferably from 70 to 100° C.

The gels obtained are neutralized for example to the extent of 0-100 mol%, preferably 25-100 mol %, particularly preferably 50-85 mol %, basedon monomer used, for which the customary neutralizing agents can beused, preferably alkali metal hydroxides, alkali metal oxides or thecorresponding alkali metal carbonates, but particularly preferablysodium hydroxide, sodium carbonate or sodium bicarbonate.

Neutralization is customarily effected by mixing in the neutralizingagent as an aqueous solution or else, preferably, as a solid. For thispurpose the gel is mechanically comminuted, by means of a mincer forexample, and the neutralizing agent is sprayed on, scattered over orpoured on and then carefully mixed in. To effect homogenization, theresultant gel mass may be passed through the mincer again a number oftimes. The neutralized gel mass is then dried with a belt dryer orroller dryer until the residual moisture content is less than 10% byweight, especially below 5% by weight. The dried hydrogel is then groundand sieved, the customary grinding apparatus being roll mills, pin millsor vibratory mills. The preferred particle size of the sieved hydrogelis preferably in the range 45-1000 μm, particularly preferably 45-850 μmmost preferably 200-850 μm.

To ascertain the quality of surface postcrosslinking, the dried hydrogelis tested using the test methods described hereinbelow:

METHODS

1) Centrifuge Retention Capacity (CRC):

This method measures the free swellability of the hydrogel in a teabag.About 0.200 g of dry hydrogel is sealed in a teabag (format: 60 mm×60mm, Dexter 1234 T paper) and soaked for min in a 0.9% strength by weightsodium chloride solution. The teabag is then spun for 3 min in acustomary commercial spindryer (Bauknecht WS 130, 1400 rpm, basketdiameter 230 mm). The amount of liquid absorbed is determined byweighing the centrifuged teabag. The absorption capacity of the teabagitself is taken into account by determining a blank value (teabagwithout hydrogel), which is deducted from the weighing result (teabagwith swollen hydrogel).

Retention CRC [g/g] =(weighing result for teabag—blank value—initialweight of hydrogel)/initial weight of hydrogel.

2) Absorbency under load (0.3/0.7 psi):

For the absorbency under load, 0.900 g of dry hydrogel is distributeduniformly on the screen base of a measuring cell. The measuring cellconsists of a Plexiglass cylinder (50 mm in height and 60 mm indiameter) whose base is formed by adhering a screen of steel mesh (meshsize 36 micron or 400 mesh). A coverplate is placed over the uniformlydistributed hydrogel and loaded with an appropriate weight. The cell isthen placed on a filter paper (S&S 589 Schwarzband, diameter=90 mm)lying on a porous glass filter plate, this filter plate itself lying ina Petri dish (30 mm in height, 200 mm in diameter) which contains 0.9%strength by weight sodium chloride solution so that the liquid level atthe beginning of the experiment is level with the top edge of the glassfrit. Hydrogel is then left to absorb the salt solution for 60 min.Subsequently the complete cell with the swollen gel is removed from thefilter plate and the apparatus is reweighed following removal of theweight.

Absorbency under load (AUL) is calculated as follows:

AUL [g/g]=(Wb-Wa)/Ws

where

Wb is the mass of the apparatus+gel after swelling,

Wa is the mass of the apparatus+initial weight of the gel beforeswelling, and

Ws is the initial weight of dry hydrogel.

The apparatus is measuring cylinder+coverplate.

EXAMPLES 1a and 1b

Base Polymer

In a 40 1 plastic bucket, 6.9 kg of glacial acrylic acid are dilutedwith 23 kg of water. 45 g of pentaerythritol triallyl ether are added tothis solution with stirring, and the sealed bucket is inertized bypassing nitrogen through it. The polymerization is then initiated byadding about 400 mg of hydrogen peroxide and 200 mg of ascorbic acid.After the reaction has ended, the gel is mechanically comminuted andadmixed with sufficient aqueous sodium hydroxide solution to provide adegree of neutralization of 75 mol %, based on the acrylic acid used.The neutralized gel is then dried on a roll dryer, ground with a pinmill and finally classified. This is the base polymer used in thesubsequent examples.

The base polymer is sprayed in a Waring lab blender with crosslinkersolution of the following composition: 4% by weight of methanol, 6% byweight of water and 0.20% by weight of N-acetyl-2-oxazolidinone, basedon polymer used. The moist polymer is then divided into two portionswhich are each heat treated at 175° C. in a through circulation cabinet,one portion for min and the other for 60 min. The dried product isclassified at 850 micron to remove lumps.

EXAMPLE 2

Base polymer as per Example 1 is sprayed with crosslinker solution in aWaring lab blender. The solution has a composition such that thefollowing dosage is obtained, based on base polymer used: 0.25% byweight of N-acetyl-2-oxazolidinone, 4% by weight of propylene glycol and6% by weight of water. The moist polymer is then dried at 175° C. for 60min in a through circulation cabinet.

EXAMPLE 3

Base polymer as per Example 1 is sprayed with crosslinker solution in aplough share lab blender. The composition of the solution is such thatthe following dosage is achieved, based on base polymer used: 0.20% byweight of N-acetyl-2-oxazolidinone, 3% by weight of methanol and 7% byweight of water. The moist polymer is then dried in a pilot plant diskdryer at 180° C. for 25 min.

EXAMPLE 4

Base polymer as per Example 1 is sprayed with crosslinker solution in aplough share lab blender. The composition of the solution is such thatthe following dosage is achieved, based on base polymer used: 0.20% byweight of N-acetyl-2-oxazolidinone, 4% by weight of 1,2-propanediol, 6%by weight of water and 0.10% by weight of boric acid. The moist polymeris then dried in a pilot plant fluidized bed dryer at 200° C. for 10min. The polymers prepared as per the above examples were tested. Theresults are reported in the table below.

TABLE Drying Drying N-Acetyl- AUL 0.3 psi AUL 0.7 psi temperature time2-oxazolidinone Catalyst Solvent CRC (g/g) (g/g) (4826.5 Pa) (g/g)Example 1 — — — — — 41 9 7 Base polymer without surface crosslinkingExample 1a 175° C. 30 min 0.20% — 4% MeOH + 35 36 19 6% H₂O Example 1b175° C. 60 min 0.20% — 4% MeOH + 30 31 25 6% H₂O Example 2 175° C. 60min 0.25% — 4% PG + 31 32 25 6% H₂O Example 3 180° C.  25 min* 0.20% —3% MeOH + 32 33 26 7% H₂O Example 4 200° C.  10 min** 0.20% 0.1% H₃BO₃4% PG + 26 28 25 6% H₂O MeOH = methanol PG = propylene glycol *Residencetime in disk dryer **Residence time in fluidized bed dryer Percentagesare by weight based on polymer used. Drying temperature and time relateto the heat treatment of the base polymer after it has been sprayed withsurface postcrosslinking solution.

We claim:
 1. A process for the gel and/or surface postcrosslinking of awater-absorbent polymer, comprising: treating said water-absorbentpolymer with a surface postcrosslinking solution, to obtain a treatedwater-absorbent polymer; postcrosslinking said treated water-absorbentpolymer to obtain a postcrosslinked water-absorbent polymer; and dryingsaid polymer during and after said treating and said crosslinking byraising a temperature; wherein the surface postcrosslinking solutioncomprises, dissolved in an inert solvent, as crosslinker a compound offormula I

wherein R¹, R², R³ and R⁴ are each independently hydrogen, C₁-C₁₂-alkyl,C₁-C₁₂-alkenyl, C₆-C₁₂-aryl or C₁-C₁₂-alkoxy, R⁵ is C₁-C₁₆-alkyl,C₁-C₄-hydroxyalkyl, C₆-C₁₂-aryl, C₁-C₁₆-alkenyl or a group of theformula —(CH₂)_(n)—COOH or —(CH₂)_(n)—SO₃H in the form of the acid or arespective metal salt, ammonium salt or triethanolammonium salt thereofor —(CH₂—CH₂—O—)_(m)—R⁶, R⁶ is hydrogen or C₁-C₁₆-alkyl, n is from 1 to10, m is from 1 to
 20. 2. The process of claim 1, wherein R⁵ is—(CH₂)_(n)—COOM or —(CH₂)_(n)—SO₃M; wherein n=1-10 and M=Na⁺, K⁺, NH₄ ⁺or (HO—CH₂—CH₂)₃NH⁺.
 3. The process of claim 1, wherein saidwater-absorbent polymer comprises structural units derived from acrylicacid or esters thereof or obtained by graft copolymerization of acrylicacid esters onto a water-soluble polymer matrix.
 4. The process of claim1, wherein surface postcrosslinking is effected using a catalystcomprising an acid or anhydride thereof.
 5. The process of claim 4,wherein surface postcrosslinking is effected using a catalyst selectedfrom the group consisting of boric acid, sulfuric acid, hydroiodic acid,phosphoric acid, tartaric acid, acetic acid, toluenesulfonic acid, anacidic salt of boric acid, an acidic salt of sulfuric acid, an acidicsalt of hydroiodic acid, an acidic salt of phosphoric acid, an acidicsalt of tartaric acid, an acidic salt of acetic acid and an acidic saltof toluenesulfonic acid.
 6. The process of claim 1, wherein said inertsolvent is water or a mixture of water and from 10 to 90% by weight of amono- or polyhydric alcohol.
 7. The process of claim 1, wherein saidcrosslinker is used in an amount of from 0.01 to 5% by weight, based ona weight of the polymer.
 8. The process of claim 1, wherein in saidcrosslinker R¹, R², R³ and R⁴ are each hydrogen and R⁵ is methyl.
 9. Awater-absorbent polymer prepared by the process of claim
 1. 10. Theprocess of claim 1, wherein said crosslinker of formula I isN-acetyl-2-oxazolidinone.
 11. The process of claim 1, wherein saidpostcrosslinking temperature occurs at a temperature of from 50 to 250°C.
 12. The process of claim 1, wherein surface postcrosslinking iseffected using a catalyst selected from the group consisting of boronoxide, sulfur trioxide, diphosphorus pentoxide, and ammoniumdihydrogenphosphate.
 13. The process of claim 1, wherein said inertsolvent is an aqueous solution of a member selected from the groupconsisting of methanol, ethanol, isopropanol, ethylene glycol,1,2-propanediol and 1,3 propanediol.
 14. The process of claim 1, whereinan amount of said surface postcrosslinking solution is 1-20% by weightbased a weight of said water-absorbent polymer.